1. Field of the Invention
This invention relates generally to the circuit and methods for configuring and operating a switching power converter. More particularly, this invention relates to an auxiliary output voltage control circuit of flyback power converter by the introduction of magnetic amplifier achieve simplified circuit, convenience of control, low production cost and precise auxiliary output voltage control.
2. Description of the Prior Art
Despite the advantages of simple circuit configuration and low production cost, conventional art of design and manufacture of a flyback converter is still limited by a technical difficulty of a larger fluctuation of auxiliary output voltage. For this reason, the flyback converters are typically implemented for systems operated at lower power below 150 watts. Additionally, the auxiliary output voltage in a flyback converter has a lower rate of adjustment relative to change of load. Due to a lower rate of adjustment to the change of the load, a linear integrating stabilizer is applied when a higher voltage stability is required for more precise control of the output voltage relative to the load changes. Such configuration is required even it is limited to system that has lower auxiliary output power. In order to overcome these limitations, more complicate circuits can be implemented and that causes a simple flyback converter to become more complicate thus losing the advantages that are inherent to a simpler configuration.
FIG. 1A is a circuit diagram for a flyback power converter where the auxiliary output is not controlled. The output voltages V10 and V20 depend on the numbers of winding n1 and n2 of the secondary side of the transformer. Specifically, the output voltages satisfy a condition:
V10/n1=V20/n2xe2x80x83xe2x80x83(1)
Since there is no feedback from the output voltages for adjusting and control, there is a poor sensitivity of the auxiliary output voltage in response to changes of the load voltage.
FIG. 1B shows a flyback power converter where a linear regulator is employed to control the auxiliary output voltage V20. Aided by the operation of the linear regulator, the auxiliary output voltage is more stable. However, such circuit configuration is suitable for application only on systems operated at a lower power range.
A magnetic amplifier is generally implemented in an auxiliary output voltage control for a forward converter as that shown in FIG. 1C. The energy transfer in a forward converter from the primary side to the secondary side carrying a load is achieved through the transformer under a condition of constant voltage. According to the duty cycle of the pulse width control circuit, a pulse width control performs the function of stabilizing the primary output voltage. When implemented with a magnetic amplifier, the forward converter has the advantages of simplified circuit and control mechanism as that shown in FIG. 1C. The forward converter with magnetic amplifier is also highly reliable with low electromagnetic interference. The function performed by the magnetic amplifier is to adjust the delay time in turning the magnetic amplifier from high impedance to low impedance by a feedback control for reset of the magnetic amplifier. The auxiliary output voltage is further adjusted by an averaging function performed by a output filtering inductor to provide stabilized output voltage. The delay time for turning magnetic amplifier to the low impedance is further controlled by a voltage-time integration of the magnetic amplifier. As the pulse width control circuit turns on the switch on the primary side, an energy transfer is achieved through the transformer under a constant voltage mode. The magnetic amplifier provides a major function of delaying the energy transfer to the auxiliary output side controlled by the magnetic amplifier feedback loop. Many disclosures and inventions have been made for implementing a magnetic amplifier in the forward converter. With a magnetic amplifier for controlling the auxiliary output voltage, excellent auxiliary output voltage adjustment to load variations are achieved. However, the forward converter has much more complicate circuit configuration than the flyback converter.
Therefore, a need still exists for those of ordinary skill in the art to provide a new and improved flyback power converter that can resolve the above-discussed technical limitations of a flyback power converter.
It is therefore an object of the present invention to provide a novel and improved flyback power converter implemented with magnetic amplifier to effectively control the auxiliary output voltage to expand the range of application of the flyback power converter such that the above-mentioned limitations can be overcome.
Specifically, magnetic amplifiers are implemented together with a pulse width modulation controller. The PWM controller is applied to control the main output voltage and the magnetic amplifiers are used to control the auxiliary output voltage in a time-shared fashion such that stabilized output voltage is produced.
A flyback power converter is disclosed in this invention. The flyback power converter includes a transformer for transferring an input voltage from a primary side to a secondary side. The secondary side includes a main output voltage loop and at least one auxiliary output loop connected in series with a magnetic amplifier. A pulse width modulation (PWM) controller controls a switch on the primary side of the transformer for turning on the switch and turning off the main output voltage loop and the auxiliary output voltage loop for storing a magnetizing energy on windings of the transformer. The PWM controller further turns off the switch and turning on the main output voltage loop for providing a main voltage output and for magnetizing the magnetic amplifier connected to the auxiliary output voltage loop. The magnetic amplifier functioning as an auxiliary-loop switch for turning on the auxiliary output voltage loop when the magnetic amplifier is saturated with magnetization and turning off the main voltage output voltage loop for providing an auxiliary output voltage. A time-shared energy transfer mechanism is provided between the main output voltage loop and the auxiliary output voltage loop to fully utilize the entire duty cycle.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the various drawing figures.